As a rule, servomotors include a permanently magnetized rotor. Two main categories of this type of motor are known, i.e.,:
dc motors PA1 stepping motors. PA1 "digital" open-loop behaviour: one fixed mechanical movement corresponds to one control pulse; PA1 positional errors are not cumulative.
In general, dc motors can be characterized by the fact that current switching is assured within the motor in such a way that the position of the rotor is at an optimum. More often than not, one strives to use solely one phase when the position of the rotor permits a high torque constant (i.e., when the torque obtained per unit of current is at a maximum). When the relative rotor-stator movement results in a reduction of the torque constant of the switched-on phase and another phase would be more efficient, the current is switched into this phase. In the case of a conventional dc motor, the collector causes the switching dependent upon the position of the rotor. In the case of a dc motor with a rotating magnet (without collector), the switching is controlled, for example, by the signals coming from a pair of Hall probes that "pick up" the position of the rotor magnet by measuring its magnetic field.
For a stepping motor, on the other hand, a sequence of switching is not "automatically" picked up by internal components of the motor. It is a logic control external to the motor that dictated the choice of the phase carrying the current, as well as the switching rhythm. Therefore, it can be said that this switching is a switching over time as opposed to a switching according to the angular position of the rotor. The advantages of the stepping motor are in particular:
U.K. Published Patent application No. 2,092,780 (corresponding to U.S. Pat. No. 4,415,844) describes a drive system for a motor without a collector, comprising Hall probes and a processor capable of processing signals of the probes in such a way that the motor rotates at a predetermined speed.
The disclosure of U.S. Pat. No. 4,376,262 describes a motor whose basic operation is of the stepping type, the pulse frequency being given by an external frequency source, but whose drive circuit comprises a speed detector and means of temporarily assuring an operation at a speed that varies continuously as it approaches the set speed, if the detector detects a variation in the effective speed in relation to the set speed.
However, it has been found that there was a need for a device capable of driving a power take-off shaft according to one's desire, either in a manner similar to that of a dc motor, or in a manner similar to that of a stepping motor.
To accomplish this, the disclosure of U.S. Pat. No. 3,940,677 proposes an arrangement in which a stepping motor and a dc motor without collector are coupled to the same shaft.
The present invention has as its object the provision of a simpler arrangement that takes up less space and is more efficient that the arrangement known from the prior art.
The aim of the invention is thus to provide a simple and compact automatic control device easy to join to a motor which will enable it to be operated alternately as a direct current motor or as a step motor, the two operating modes able to succeed each other without any intermediate stoppage of the motor.
To that end, the object of the present invention is a drive device for an electric motor of which the stator and rotor include for one of them a motor winding of one or several phases and for the other of them a permanently magnetized magnetic element, with one or several pairs of poles. The device comprises a clock signal generator, an orientation signal generator indicating the orientation of the rotor at successive instants, control means capable of controlling the triggering and release of the phase or phases of the winding, in a first mode of operation as a function of the clock signals, and in a second mode of operation as a function of the orientation signals, and the selection means making it possible to put into operation one of the operating modes.